One partner being insecure and not actively resolving their own insecurity issues, ruins monogomous relationships on a regular basis.....
So it doesn't make a sound argument (imho) for why one is or is not poly.
NOW that said-I agree that insecurity is a problem in poly relationships, but no more or less then it is in mono relationships.
The major difference I see (first hand) is that in a poly relationship there are more people dealing with it and so it tends to be pointed out to you more often.
But the truth is -NO relationship is healthy AT ALL until trust is established AND personal insecurities have been addressed BY YOURSELF.
I HIGHLY advise checking out www.xeromag.com
and reading his articles on polyamory-and specifically the ones on insecurity. Every person ought to consider reading them-not just the poly people.
EVEN if your relationship remains monogomous-it will be improved if you learn how to manage your own personal issues.
As for BDSM-there are several threads on here that make it quite clear that TRUST is IMPERATIVE in a BDSM relationship of any type-I would HIGHLY advise that you understand-if you two are allowing in your relationship for her to have another partner in BDSM, that she DOES need to feel a level of trust with them at least equal to what she has with you if not more so AND that you need to able and willing to accept whatever it takes for that level of trust to be built OR she is endangering herself getting into a relationship like that.